A sterilization process used to sterilize medical and hospital equipment is only effective if a certain combination of environmental conditions is achieved within the sterilization chamber of the sterilizer. For example, when steam is used as a sterilant, the object of the sterilization process is to bring steam of a suitable quality, and at an appropriate temperature into contact with all surfaces of the articles being sterilized for a correct length of time
In some steam sterilizers the process of sterilization is typically conducted in three main phases. In the first phase, air trapped within the load being processed is removed. The second phase is a sterilizing stage, in which the load is subjected to steam under pressure for a recognised combination of time and temperature, which is known to effect sterilization. The third phase is a drying phase in which condensate formed during the first two phases is removed by evacuating the chamber.
Air removal from the sterilization chamber may be achieved in a number of ways. For example, in a gravity steam sterilizer, the principle of gravity displacement is utilized, in which steam entering at the top of the chamber displaces the air through a valve in the base of the chamber. In a prevacuum steam sterilizer, on the other hand, air is removed forcibly by deep evacuation of the chamber or by a combination of evacuation and steam injection at either subatmospheric and/or superatmospheric pressures.
Any air which is not removed from the sterilization chamber during the air removal phase of the cycle or which leaks into the chamber during a subatmospheric pressure stage due to faulty gaskets, valves or seals, may form air pockets within the load that is being sterilized. Likewise, any non-condensable gases (which, in this context, means gases having a boiling point below that of the sterilant) that are present in the sterilization chamber or are carried within steam supplied to the chamber may form gas pockets within the load. These air or gas pockets will create a barrier to steam penetration, thereby preventing adequate sterilizing conditions being achieved for all surfaces of the load. This is particularly true when porous materials such as hospital linens or fabrics are being sterilized since the air or gas pockets prohibit the steam from penetrating to the interior layers of such materials. As a result, sterilization may not occur. Therefore, there is a need to be able to determine the efficacy of sterilization cycles and in particular, to determine whether there has been sufficient steam penetration. Similarly, when a sterilant other than steam is used, there is a need to be able to determine that the sterilant has penetrated a load sufficiently for sterilization to take place.
One commonly-used procedure for evaluating the effectiveness of air removal during the air removal phase of a porous load steam sterilization cycle and/or for testing for the presence of non-condensable gases is known as the Bowie-Dick test. The typical Bowie-Dick test pack essentially consists of a stack of freshly laundered towels folded to a specific size, with a chemical indicator sheet placed in the centre of the pack. Chemical indicator test sheets undergo a visible change from one distinct colour to another, for example, from an initial white to a final black colour, upon exposure to the sterilization process. If the air removal within the sterilizer is insufficient, or if non-condensable gases are present during the process in sufficient quantity, an air/gas pocket will form in the centre of the pack thereby preventing steam from contacting the steam sensitive chemical indicator sheet. The consequence of inadequate steam penetration is a non-uniform colour development across the surface of the chemical indicator test sheet: thus, the presence of the air/gas pocket will be recorded by the failure of the indicator to undergo the complete or uniform colour change indicative of adequate steam penetration.
Biological indicators can also be used to provide information on the adequacy of a sterilization cycle. Biological indicator test systems typically employ living spores which are subjected to a sterilization cycle. After the cycle, the spores are incubated and the system detects if there is any growth. If there is no growth, it indicates that the sterilization process has been effective. Thus, biological indicators can determine whether conditions for sterilization were present, but the length of time to obtain results due to the incubation period is often at least 24 hours. Therefore, biological indicator systems are often used in conjunction with chemical indicators because the colour change of the chemical indicators provides an instant result. Further, by using both chemical and biological indicators, information on both the adequacy of the air removal stage and the sterilization stage is provided.
Parametric monitoring has also been used to either monitor or control a sterilization cycle to ensure proper sterilization conditions are attained. For example, in U.S. Pat. No. 4,865,814 to Childress, an automatic sterilizer is disclosed which includes a microprocessor which monitors both the temperature and pressure levels inside the sterilization chamber and controls a heater to allow both pressure and temperature to reach predetermined levels before starting a timer. Once the timer is started, it is stopped if the pressure or temperature levels drop below a predetermined minimum. Since it is known that the pressure and temperature variables of saturated steam are dependent variables when saturated steam is enclosed in a sealed chamber, monitoring of these two variables can ensure that proper conditions are maintained during the sterilization cycle.
Although it is desirable to monitor environmental conditions within the sterilization chamber itself, it is generally considered more desirable to be able to monitor the environmental conditions within an actual load being sterilized or within a test pack (such as the Bowie-Dick test pack) that represents such a load. Although the typical Bowie-Dick test pack is generally recognized as adequate for use in determining the efficacy of the air removal stage of prevacuum sterilizers, it still presents many disadvantages. Since the test pack is not preassembled, it must be constructed every time the procedure is used to monitor sterilizer performance. The preparation, assembly and use of the towel pack is time consuming and cumbersome and, moreover, varying factors, such as laundering, prehumidification, towel thickness and wear, and the number of towels used, alter the test results. Therefore, alternative Bowie-Dick test packs have been developed to overcome these limitations.
An example of an alternative Bowie-Dick test pack for steam or gas sterilizers is described in EP-A-0419282. That test pack includes a container having top and bottom walls with a porous packing material disposed within the container. The packing material challenges the penetration of the sterilant by providing a restricted pathway which acts to impede the flow of the sterilant through the test pack. A removable lid seals the bottom end of the container, while a hole in the top wall of the container allows for the downward ingress of steam into the packing material within the container. The test pack includes a chemical indicator for detecting sterilant penetration. If sterilant successfully penetrates the packing material of the test pack, the chemical indicator sheet will undergo a complete colour change. If the sterilant does not sufficiently penetrate the packing material, the chemical indicator will not undergo a complete uniform colour change, thereby indicating inadequate air removal or the presence of non-condensable gas, or in other words, a Bowie-Dick test failure.
Other test packs for use in steam or gas sterilizers are described in EP-A-0 421 760; U.S. Pat. No. 5,066,464; WO 93/21964 and U.S. Pat. No. 5,270, 217. In each of those test packs, sterilant from the sterilization chamber must cross some form of physical barrier before it reaches a sterilant sensor within the test pack. WO 93/21964, for example, describes a test unit comprising a test cavity having an opening at one end to permit entrance of ambient gases, a temperature sensor at the other end and a heat sink (for example gauze, felt, open-celled polymer foam) between the temperature sensor and the opening.
U.S. Pat. No. 4,594,223 describes various devices for indicating the presence of non-condensable gas in a sterilization chamber. In one version, a heat and humidity sensor is located at the lower end of an elongate cavity which is open at the upper end. Heat sink material in the form of fibrous insulating material is located within the cavity between the opening and the sensor. In another version, the path between the opening and the sensor is through a heat sink block in the form of a mass of aluminum surrounded by insulation, rather than through fibrous heat sink material. U.S. Pat. No. 4,115,068 describes an air indicating device for use in sterilizers, comprising an upright tube which is open at its bottom end and closed at its top end. The tube is made of heat insulating material lined on its interior surface with a heat conducting material. A thermal indicator strip extends axially into the tube.
Another known arrangement for challenging the penetration of sterilant to a particular location within a test pack comprises a very long (typically, 1.5 m) stainless steel tube with a narrow bore (typically, 2.0 mm) which provides the only access for sterilant to the predetermined location.